As Bill says, does the door have a latch on it with a turn handle? The
latch maybe stuck out and the handle is just turning around
doing nothing. Try slipping a strip off a plastic bottle into the
gap at the latch to release it.

Tried that but can't get anything to go all the way in round the
'corner' of the door frame...

I tried again and DID manage to slip a credit card all the way into the door
frame so I could slide it up and down. It won't let me slide where the
latch is though. I've used pliers to turn the handle as far as it will go
but, checking with the credit card, that latch is still very much sticking
out so it looks like that's definitely the problem... I can't shove the
card in where it needs to go though to force hte latch back in no matter how
hard I try...